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Basic Concepts of GIS January 29, 2013

Basic Concepts of GIS January 29, 2013. What is GIS? “A powerful set of tools for collecting, storing, retrieving, transforming and displaying spatial data”. A Geographic Information System requires: (1) Data containing locational information

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Basic Concepts of GIS January 29, 2013

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  1. Basic Concepts of GISJanuary 29, 2013

  2. What is GIS?“A powerful set of tools for collecting, storing, retrieving, transforming and displaying spatial data”

  3. A Geographic Information System requires: (1) Data containing locational information (2) An ability to store or readily compute the spatial interrelations that describe how the data are connected (also known as topology) Having information about how the data was collected, its precision and accuracy (metadata) is preferable, but not essential.

  4. Topology Computational geometry – branch of mathematics that deals with the spatial relationships between and among objects All features/objects are one of the following types: 0 dimensions - point (node) 1 dimension - line (arc, link) 2 dimensions - area (polygon) 3 dimensions - volume

  5. Spatial relations example

  6. Examples of Spatial/Topological Relations: Belongs (New Castle, Kent and Sussex belong to Delaware) Comprises (Delaware comprises New Castle, Kent and Sussex) Located in (The city of Wilmington is located in New Castle) Borders on (New Castle borders on Kent but not Sussex) Is a certain distance from (Wilmington is 47 miles from Dover) Is within a certain distance of (Dover is within 60 miles of every point in the state of Delaware)

  7. Until about 1995, a GIS stored topological information explicitly: Node topology - a listing of nodes and the links that meet at each node Link topology - a listing of links and the nodes that bound them, as well as the polygons to the left and right of each link Polygon topology - a listing of polygons and the links that define them Since that time, topological information is not stored, but calculated on the fly using various algorithms (more on this later!)

  8. The shape file has long been the standard file type for GIS information Consists of a minimum of three parts: - .shp – the feature geometry itself - .shx – shape index format; a positional index to allow seeking forwards and backwards quickly - .dbf – the data attributes A shape file contains a collection of points, lines, or polygons – never a mixture!

  9. GIS vs. non-GIS Find the distance from Albany to Buffalo: GIS program, or Google Earth: Simple calculation executable through a drop-down menu Non-GIS: Obtain the coordinates for Albany and Buffalo. Type in the Pythagorean distance formula. If your coordinates are in latitude and longitude, add cosine terms to account for the curvature of the earth. Add additional trigonometric terms to account for the non-spherical nature of the earth.

  10. Find the counties that border Albany County: GIS program: Simple calculation executable through a drop-down menu Regular database (brute force approach): Create a table of all coordinate points defining the borders of every county in New York. Match Albany county coordinates against all county coordinates. (An exact match is not good enough if coverage is not clean – or if the Hudson River is stored as a separate object – you would then need to write code to find points that are within some small distance of one another.) Google Earth: No obvious solution

  11. 2 Broad Models of Reality: 1. Space is occupied by entities: real-world phenomena that are not divisible into phenomena of the same kind. Examples: County Building Disease case 2. Space is occupied by continuously varying attributes. Examples: Air pollution Temperature Elevation Vector vs. raster

  12. 2 Broad Models of Reality: 1. Space is occupied by entities: real-world phenomena that are not divisible into phenomena of the same kind. Examples: County Building Disease case 2. Space is occupied by continuously varying attributes. Examples: Air pollution Temperature Elevation Vector model Raster model

  13. Vector and raster representations of a river:

  14. Some basic GIS functions: Distance Perimeter Area Centroid

  15. Centroid

  16. Distance

  17. Area and Perimeter Area and Perimeter

  18. Centroid

  19. Some more basic GIS functions: Intersect Buffer Polygon Overlay Combine/Split Objects

  20. Intersect Select all from SchoolDist, ZIP where SchoolDist.obj intersects ZIP.obj

  21. Buffer

  22. Buffer +Intersect

  23. Polygon Overlay

  24. Combine/split objects

  25. Advanced functions Advanced Spatial Analysis Functions in GIS: Shortest Path Routing Location-Allocation Nearest Neighbor Spatial autocorrelation Spatial interpolation

  26. Shortest path

  27. Routing

  28. Location-Allocation

  29. Nearest neighbor

  30. Spatial autocorrelation High None

  31. Spatial interpolation + + + + + + + + + + + + + + + +

  32. Now to look at some algorithms… Point in polygon: Ray-tracing and the even-odd rule

  33. Many points in many polygons: The grid-based method

  34. Polygon intersection: The bounding-box rectangle method

  35. Shortest path: Dykstra’s algorithm

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